The invention relates to electric powered tools, and more particularly, to a dynamic brake electrical circuit for use in stopping an alternating current universal electrical motor used in power tools.
Alternating current (a.c) universal motors have been commonly used in electrically powered tools such as power saws, drills, and other types of equipment. Generally they are a high-power, light weight power source for electrical tools. The typical universal motor has a housing supporting a stator assembly and a rotatable arbor or shaft which has an armature mounted thereon. Current flowing through the run coil on the stator creates a magnetic field which interacts with the armature magnetic field which in turn drives the arbor rotationally. A tool such as a saw blade or drill bit may be mounted directly on the arbor or the arbor may be coupled to the tool by a simple gear transmission or the like.
Those skilled in the art will appreciate that universal motors have a tendency to coast, or to continue rotation of the arbor shaft for some time after electrical power is disconnected from the motor. This tendency to coast is due simply to the rotational momentum of the arbor shaft, transmission, and tool attachments. For this reason, it is sometimes desirable to equip the motor with a brake. One such brake is a regenerative dynamic discharging capacitor was to generate, and thereby guarantee the existence of, a residual magnetism in the stator iron to help assure adequate braking action. The addition of the discharge capacitor helps eliminate the "dead-zone" but can create other problems. For example, if the break coil leads break, or, more likely, the commutator brushes are worn and not in contact with the armature, the capacitor remains charged for some time creating a shock hazard to anyone attempting to service the tool. While the capacitor may be manually discharged, for example, by shorting the capacitor leads with a screwdriver blade, service personnel do not always observe this precaution. Moreover, shorting the capacitor leads in such a manner may destroy the component or decrease its effective life.
An additional problem is that conventional dynamic braking circuits have required complicated switching arrangements which necessitate more expensive and complex switches. A simplified circuit design allows the use of a less expensive switch having fewer leads and moveable parts and, therefore, having a greater degree of reliability. Thus, it is desirable to provide a dynamic braking circuit which is reliable, and inexpensive to manufacture.